The determination of the blade angle of propulsion system blades is critical for controlling today's propeller and other multi-bladed fan propulsion systems used in aircraft and other vehicles. In typical systems known in the art, blade angle measurements are taken at some remote location instead of directly from the actual blades. Typically, this remote position is found in the mechanism used for adjusting blade angle. Moreover, due to aggregate tolerances in mechanical systems, disconnections, and malfunctions within such mechanical systems, such a remote measurement of blade angle can, in many cases, be incorrect. With an incorrect reading on blade angle, further adjustment of blade angle based on these readings either manually or by a control system, could cause the propulsion system to enter an unwanted mode, such as feather or flat pitch, when such a mode is not conducive to safe operation given the particular flight conditions. Accordingly, the only sure manner to avoid originating blade angle adjustments from an incorrect base reading, is to directly measure the angle of the propulsion system blades. However, a reliable application of such a system, prior to this invention, has yet to be discovered.
The prior art includes a plurality of blade angle systems, all of which suffer from at least one of the defects discussed above.
For example, U.S. Pat. No. 2,370,167 to Hoover et al. discloses an indicator for variable pitch propellers. The mechanism includes a drive shaft, a part connected to the shaft for rotation therewith, and a part normally rotating with the shaft capable of rotation at a speed different from that of the shaft. Means responsive to differential rotation of the two parts is also included for adjusting the pitch of a blade carried by the shaft. Also, means for causing differential rotation of the parts and means for indicating pitch of the blade are also included. The means for indicating the pitch of the blade comprises means surrounding the shaft for setting up a magnetic field. The mechanism further includes means carried by the shaft and rotatable therewith in the magnetic field for producing an alternating current of a frequency proportional to the speed of rotation of the shaft. Means carried by the second named part and rotatable therewith in the magnetic field produces an alternating current of an alternating frequency, proportional to the speed of rotation of the second named part. Finally, means for producing an indication of the amount of any variation between the frequencies of the two alternating currents is provided. Accordingly, in this mechanism, the blade angle is determined through measurement of discrete parts and is not determined directly from measurements taken from the blade potentially leading to false readings.
U.S. Pat. No. 5,211,539 to McCarty discloses an apparatus for indicating the pitch of turbo fan blades. The apparatus indicates changes in blade pitch of variable pitch blades by detecting relative displacements of first and second members of a blade pitch actuator. The members are selectively displaced to change the pitch of the blades. The apparatus includes sensors which provide output signals to a microprocessor, and which determines displacement of each member with respect to a frame of reference. The microprocessor takes the difference between a displacement of the two members to determine the relative displacement, which is indicative of a change in blade pitch. As indicated above, an inherent problem with such a design is that the pitch determination is dependent upon positions of members comprising the blade pitch actuator and not the actual blade. Accordingly, if a malfunction occurs where the blade is somehow partially or fully disconnected from the actuator, readings from the pitch indicator disclosed herein will provide incorrect information as to the actual pitch of the blades. Accordingly, corrective measures or other adjustments could place the blades into a highly undesirable pitch angle.
U.S. Pat. No. 2,485,245 to Schonembaum discloses a control system for use in controlling the pitch angles of propeller blades. The pitch indicating system, in combination with an aircraft housing, a propeller shaft extending therethrough, a hub mounted on a shaft and variable pitch propeller blades journaled to the hub, includes control members rotatable with the propeller system and moveable relative to each other in correspondence with the pitch angle of the propeller blades. Means energized by one of the control members for part of each revolution of the propeller is provided along with means energized by another of the control members for the remaining part of each revolution of the propeller. Means proportionally responsive to the difference in average energization of the first named means and the second named means is also used. Specific attention is directed to the FIGS. 4 and 5 embodiments, wherein bars 44 are used with propeller blades for sensing by magnet 42. The bar 44 and a structure 45 are the control members which move relative to each other when the pitch of the propeller blades is changed. The bar is shaped such that for any pitch position of the propeller blade, there is substantially no change in the length of the air gap between the surface of the bar and the end of the magnetic pole pieces, when the bar is in an armature forming position. Based on the dual sensing of the bar 44 and of structure 45, the pitch angle can be determined. Since the use of only one bar 44 is shown and taught, the length of bar 44 is problematic for current propeller designs, substantially enhancing the possibility of damaging the bar and also requiring large openings in the propeller bulkhead for sensing by sensors. Accordingly, since such a design is highly subject to damage, the usefulness of the same and current propeller systems is highly suspect.
There exists a need, therefore, for an improved direct blade angle measurement system wherein blade angle is measured directly from propulsion system blades, which system does not require components which are prohibitively long as with current propeller designs, thereby decreasing the likelihood of system component damage.